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<a name="Stack-Arguments"></a>
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Next: <a href="Register-Arguments.html#Register-Arguments" accesskey="n" rel="next">Register Arguments</a>, Previous: <a href="Elimination.html#Elimination" accesskey="p" rel="prev">Elimination</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Passing-Function-Arguments-on-the-Stack"></a>
<h4 class="subsection">18.9.6 Passing Function Arguments on the Stack</h4>
<a name="index-arguments-on-stack"></a>
<a name="index-stack-arguments"></a>

<p>The macros in this section control how arguments are passed
on the stack.  See the following section for other macros that
control passing certain arguments in registers.
</p>
<dl>
<dt><a name="index-TARGET_005fPROMOTE_005fPROTOTYPES"></a>Target Hook: <em>bool</em> <strong>TARGET_PROMOTE_PROTOTYPES</strong> <em>(const_tree <var>fntype</var>)</em></dt>
<dd><p>This target hook returns <code>true</code> if an argument declared in a
prototype as an integral type smaller than <code>int</code> should actually be
passed as an <code>int</code>.  In addition to avoiding errors in certain
cases of mismatch, it also makes for better code on certain machines.
The default is to not promote prototypes.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fPUSH_005fARGUMENT"></a>Target Hook: <em>bool</em> <strong>TARGET_PUSH_ARGUMENT</strong> <em>(unsigned int <var>npush</var>)</em></dt>
<dd><p>This target hook returns <code>true</code> if push instructions will be
used to pass outgoing arguments.  When the push instruction usage is
optional, <var>npush</var> is nonzero to indicate the number of bytes to
push.  Otherwise, <var>npush</var> is zero.  If the target machine does not
have a push instruction or push instruction should be avoided,
<code>false</code> should be returned.  That directs GCC to use an alternate
strategy: to allocate the entire argument block and then store the
arguments into it.  If this target hook may return <code>true</code>,
<code>PUSH_ROUNDING</code> must be defined.
</p></dd></dl>

<dl>
<dt><a name="index-PUSH_005fARGS_005fREVERSED"></a>Macro: <strong>PUSH_ARGS_REVERSED</strong></dt>
<dd><p>A C expression.  If nonzero, function arguments will be evaluated from
last to first, rather than from first to last.  If this macro is not
defined, it defaults to <code>PUSH_ARGS</code> on targets where the stack
and args grow in opposite directions, and 0 otherwise.
</p></dd></dl>

<dl>
<dt><a name="index-PUSH_005fROUNDING"></a>Macro: <strong>PUSH_ROUNDING</strong> <em>(<var>npushed</var>)</em></dt>
<dd><p>A C expression that is the number of bytes actually pushed onto the
stack when an instruction attempts to push <var>npushed</var> bytes.
</p>
<p>On some machines, the definition
</p>
<div class="smallexample">
<pre class="smallexample">#define PUSH_ROUNDING(BYTES) (BYTES)
</pre></div>

<p>will suffice.  But on other machines, instructions that appear
to push one byte actually push two bytes in an attempt to maintain
alignment.  Then the definition should be
</p>
<div class="smallexample">
<pre class="smallexample">#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) &amp; ~1)
</pre></div>

<p>If the value of this macro has a type, it should be an unsigned type.
</p></dd></dl>

<a name="index-outgoing_005fargs_005fsize"></a>
<a name="index-crtl_002d_003eoutgoing_005fargs_005fsize"></a>
<dl>
<dt><a name="index-ACCUMULATE_005fOUTGOING_005fARGS"></a>Macro: <strong>ACCUMULATE_OUTGOING_ARGS</strong></dt>
<dd><p>A C expression.  If nonzero, the maximum amount of space required for outgoing arguments
will be computed and placed into
<code>crtl-&gt;outgoing_args_size</code>.  No space will be pushed
onto the stack for each call; instead, the function prologue should
increase the stack frame size by this amount.
</p>
<p>Setting both <code>PUSH_ARGS</code> and <code>ACCUMULATE_OUTGOING_ARGS</code>
is not proper.
</p></dd></dl>

<dl>
<dt><a name="index-REG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>REG_PARM_STACK_SPACE</strong> <em>(<var>fndecl</var>)</em></dt>
<dd><p>Define this macro if functions should assume that stack space has been
allocated for arguments even when their values are passed in
registers.
</p>
<p>The value of this macro is the size, in bytes, of the area reserved for
arguments passed in registers for the function represented by <var>fndecl</var>,
which can be zero if GCC is calling a library function.
The argument <var>fndecl</var> can be the FUNCTION_DECL, or the type itself
of the function.
</p>
<p>This space can be allocated by the caller, or be a part of the
machine-dependent stack frame: <code>OUTGOING_REG_PARM_STACK_SPACE</code> says
which.
</p></dd></dl>

<dl>
<dt><a name="index-INCOMING_005fREG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>INCOMING_REG_PARM_STACK_SPACE</strong> <em>(<var>fndecl</var>)</em></dt>
<dd><p>Like <code>REG_PARM_STACK_SPACE</code>, but for incoming register arguments.
Define this macro if space guaranteed when compiling a function body
is different to space required when making a call, a situation that
can arise with K&amp;R style function definitions.
</p></dd></dl>

<dl>
<dt><a name="index-OUTGOING_005fREG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>OUTGOING_REG_PARM_STACK_SPACE</strong> <em>(<var>fntype</var>)</em></dt>
<dd><p>Define this to a nonzero value if it is the responsibility of the
caller to allocate the area reserved for arguments passed in registers
when calling a function of <var>fntype</var>.  <var>fntype</var> may be NULL
if the function called is a library function.
</p>
<p>If <code>ACCUMULATE_OUTGOING_ARGS</code> is defined, this macro controls
whether the space for these arguments counts in the value of
<code>crtl-&gt;outgoing_args_size</code>.
</p></dd></dl>

<dl>
<dt><a name="index-STACK_005fPARMS_005fIN_005fREG_005fPARM_005fAREA"></a>Macro: <strong>STACK_PARMS_IN_REG_PARM_AREA</strong></dt>
<dd><p>Define this macro if <code>REG_PARM_STACK_SPACE</code> is defined, but the
stack parameters don&rsquo;t skip the area specified by it.
</p>
<p>Normally, when a parameter is not passed in registers, it is placed on the
stack beyond the <code>REG_PARM_STACK_SPACE</code> area.  Defining this macro
suppresses this behavior and causes the parameter to be passed on the
stack in its natural location.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fRETURN_005fPOPS_005fARGS"></a>Target Hook: <em>poly_int64</em> <strong>TARGET_RETURN_POPS_ARGS</strong> <em>(tree <var>fundecl</var>, tree <var>funtype</var>, poly_int64 <var>size</var>)</em></dt>
<dd><p>This target hook returns the number of bytes of its own arguments that
a function pops on returning, or 0 if the function pops no arguments
and the caller must therefore pop them all after the function returns.
</p>
<p><var>fundecl</var> is a C variable whose value is a tree node that describes
the function in question.  Normally it is a node of type
<code>FUNCTION_DECL</code> that describes the declaration of the function.
From this you can obtain the <code>DECL_ATTRIBUTES</code> of the function.
</p>
<p><var>funtype</var> is a C variable whose value is a tree node that
describes the function in question.  Normally it is a node of type
<code>FUNCTION_TYPE</code> that describes the data type of the function.
From this it is possible to obtain the data types of the value and
arguments (if known).
</p>
<p>When a call to a library function is being considered, <var>fundecl</var>
will contain an identifier node for the library function.  Thus, if
you need to distinguish among various library functions, you can do so
by their names.  Note that &ldquo;library function&rdquo; in this context means
a function used to perform arithmetic, whose name is known specially
in the compiler and was not mentioned in the C code being compiled.
</p>
<p><var>size</var> is the number of bytes of arguments passed on the
stack.  If a variable number of bytes is passed, it is zero, and
argument popping will always be the responsibility of the calling function.
</p>
<p>On the VAX, all functions always pop their arguments, so the definition
of this macro is <var>size</var>.  On the 68000, using the standard
calling convention, no functions pop their arguments, so the value of
the macro is always 0 in this case.  But an alternative calling
convention is available in which functions that take a fixed number of
arguments pop them but other functions (such as <code>printf</code>) pop
nothing (the caller pops all).  When this convention is in use,
<var>funtype</var> is examined to determine whether a function takes a fixed
number of arguments.
</p></dd></dl>

<dl>
<dt><a name="index-CALL_005fPOPS_005fARGS"></a>Macro: <strong>CALL_POPS_ARGS</strong> <em>(<var>cum</var>)</em></dt>
<dd><p>A C expression that should indicate the number of bytes a call sequence
pops off the stack.  It is added to the value of <code>RETURN_POPS_ARGS</code>
when compiling a function call.
</p>
<p><var>cum</var> is the variable in which all arguments to the called function
have been accumulated.
</p>
<p>On certain architectures, such as the SH5, a call trampoline is used
that pops certain registers off the stack, depending on the arguments
that have been passed to the function.  Since this is a property of the
call site, not of the called function, <code>RETURN_POPS_ARGS</code> is not
appropriate.
</p></dd></dl>

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